Both polyamides and polyolefins have, for many years, been commonly used to form articles of all sizes and shapes. Each group of materials has its own advantages and drawbacks. More specifically, polyamides yield products having high rigidity and good resistance to many corrosive chemicals. However, they are subject to hydrolysis and possess a limited barrier effect against penetration by moisture. Moreover, their characteristics are such that, even if they are modified by elastomers, plasticization is possible only to a limited extent. On the other hand, polyolefins can be readily plasticized, have a high resistance to hydrolysis, and possess an excellent barrier to moisture or humidity.
Since there are many mechanical and electrical components which comprise pluralities of functional parts, it is not surprising to note that in many cases, a combination of flexible and rigid materials is required. Often, a flexible membrane is integrated into a rigid housing. At present, this is usually accomplished by screw connections or adhesive. However, methods of this type require hand labor and are time consuming and expensive.
A special injection welding technique is described in an article by R. Hagen in "Kunststoffe", 79 (1989), on pages 72 to 76. In accordance with this method, parts of compatible polymers can be joined without screws or adhesives. However, if one attempts this technique with incompatible thermoplastic polymers, they do not adhere to one another very well at their interfaces. Thus, some additional mechanical or other form of fixation is required. Moreover, the interfaces are not tight to either liquids or gases.
Thus, the injection welding and extrusion techniques will permit attachment of two thermoplastics to one another only if they have the same or a similar chemical structure. Moreover, even if two such similar materials are simultaneously injection molded or coextruded, mutual adhesion is obtained only if the thermoplastic with the higher softening or melting point is injected or extruded onto the one with a lower softening or melting point. If this technique is not followed, adhesion at the interfaces is generally poor.